Unsymmetrical photodimerization of a 9-aminomethylanthracene in the crystalline salt

Article abstract of DOI:10.1021/jo060315i

By the salt formation with particular nonaromatic dicarboxylic acids, rapid and selective photodimerization of 9-(N,N-dimethylaminomethyl)anthracene (1) was accomplished in the solid state. For instance, the salt with trans,trans-muconic acid or acetylenedicarboxylic acid was led quantitatively to the 9,10:4′,1′ photodimer usy-ht-2, the first example of the unsymmetrical [4+4] photodimerization of anthracene in the solid state. The reactions were rationalized by the relevant C...C distances between the reacting carbons.

Full text of DOI:10.1021/jo060315i

SCHEME 1. Previous Reaction Reported by Ihmels et al.5b
Unsymmetrical Photodimerization of a
9-Aminomethylanthracene in the Crystalline Salt
Masahiro Horiguchi and Yoshikatsu Ito*
Department of Synthetic Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto UniVersity,
Katsura, Kyoto 615-8510, Japan
yito@sbchem.kyoto-u.ac.jp
its crystalline salt, with a certain aromatic carboxylic acid (i.e.,
2
-furancarboxylic acid) is photoreactive to give the head-to-
ReceiVed February 15, 2006
5
b
tail dimer ht-2 (Scheme 1). It seems to us, however, that the
aromatic carboxylic acid is generally not a good candidate to
induce the photodimerization of 1 in crystal, because a potential
2
b
herringbone or stacking interaction of the aromatic ring of
the acid with the anthracene ring of 1 will disturb its photo-
dimerization. Here we will report our recent study on controlling
the solid-state photodimerization of 1 by using nonaromatic
dicarboxylic acids. The R,ω-dicarboxylic acids of symmetric
structures afforded good salt crystals with 1 relatively easily.
We employed six dicarboxylic acids, oxalic acid (ox), succinic
acid (su), adipic acid (ad), fumaric acid (fu), trans,trans-
muconic acid (mu), and acetylenedicarboxylic acid (ac). Each
crystalline salt was obtained by the recrystallization of a 1:0.5
By the salt formation with particular nonaromatic dicar-
boxylic acids, rapid and selective photodimerization of
(molar ratio) mixture of 1 and dicarboxylic acid from MeOH
or i-PrOH. As summarized in Table 1, crystalline salts with
the 1:0.5 or 1:1 stoichiometry of amine/dicarboxylic acid were
obtained. Salt 1ac crystallized out with or without inclusion of
methanol (1ac‚MeOH or 1ac, respectively), depending on the
solvent used. The composition of the 1/acid in the salt was
9
-(N,N-dimethylaminomethyl)anthracene (1) was accom-
plished in the solid state. For instance, the salt with
trans,trans-muconic acid or acetylenedicarboxylic acid was
led quantitatively to the 9,10:4′,1′ photodimer usy-ht-2, the
first example of the unsymmetrical [4+4] photodimerization
of anthracene in the solid state. The reactions were rational-
ized by the relevant C‚‚‚C distances between the reacting
carbons.
1
determined on the basis of the H NMR and elemental analysis.
Salt formation was ascertained with the IR spectroscopy from
which the carbonyl absorption of the carboxylate anion was
observed at 1560-1624 (asymmetric stretch) and 1326-1405
-
1
cm (symmetric stretch).
First, as a control experiment, solution photolysis of 1 was
conducted with 0.018 M of 1 in MeOH under an Ar atmosphere
at 5 °C. Irradiation was performed with a 400-W high-pressure
mercury lamp through a Pyrex filter. As shown in Scheme 2,
Recently, the study on supramolecular photochemistry has
been attracting broad attention. We are exploiting supramo-
1
lecular two-component crystals with the intention of increasing
2
the variety and diversity of solid-state photochemical reactions,
which is still lacking as compared with that in solution.
6
3
free amine 1 yielded ht-2 together with lepidopterene (3) and
4
. The formation of 3 and 4 was explained previously by
Photodimerization of anthracenes has been studied both in
4
5
considering, respectively, head-to-tail and head-to-head coupling
solution and in the solid state. For example, Ihmels et al. have
reported that the solid-state irradiation of 9-(N,N-dimethylami-
nomethyl)anthracene (1) does not result in a photoreaction, while
(
4) (a) Bouas-Laurent, H.; Castellan, A.; Desvergne, J.-P.; Lapouyade,
R. Chem. Soc. ReV. 2001, 30, 248-263. (b) Becker, H. D. Chem. ReV.
993, 93, 145-172. (c) Bouas-Laurent, H.; Desvergne, J.-P. Photochromism;
1
(
1) (a) Huang, C.-H.; Bassani, D. M. Eur. J. Org. Chem. 2005, 4041-
Molecules and Systems; D u¨ rr, H., Bouas-Laurent, H., Eds.; Elsevier:
Amsterdam, The Netherlands, 1990; Chapter 4, pp 561-630. (d) Bouas-
Laurent, H.; Castellan, A.; Desvergne, J.-P. Pure Appl. Chem. 1980, 52,
2633-2648.
4
4
050. (b) Karthikeyan, S.; Ramamurthy, V. Tetrahedron Lett. 2005, 46,
495-4498 and references therein.
(2) (a) Ito, Y. Synthesis 1998, 1-32. (b) Ito, Y. In Organic Molecular
Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker:
New York, 1999; pp 1-70. (c) Ito, Y.; Kitada, T.; Horiguchi, M.
Tetrahedron 2003, 59, 7323-7329. (d) Ito, Y.; Horie, S.; Shindo, Y. Org.
Lett. 2001, 3, 2411-2413. (e) Ito, Y.; Hosomi, H.; Ohba, S. Tetrahedron
(5) (a) Schmidt, G. M. J. Pure Appl. Chem. 1971, 27, 647-678. For a
review, see, for example, ref 3a. (b) Ihmels, H.; Leusser, D.; Pfeiffer, M.;
Stalke, D. Tetrahedron 2000, 56, 6867-6875. (c) Ito, Y.; Olovsson, G. J.
Chem. Soc., Perkin Trans. 1 1997, 127-133. (d) Ito, Y.; Fujita, H. J. Org.
Chem., 1996, 61, 5677-5680. (e) Ito, Y. Mol. Cryst. Liq. Cryst. Sci.
Technol., Sect. A 1996, 277, 247-253.
2
000, 56, 6833-6844. (f) Ito, Y.; Fujita, H. Chem. Lett. 2000, 288-289.
3) (a) Ramamurthy, V.; Venkatesan, K. Chem. ReV. 1987, 87, 433-
(
481. (b) Scheffer, J. R.; Garcia-Garibay, M.; Nalamasu, O. Org. Photochem.
1987, 8, 249-347. (c) Singh, N. B.; Singh, R. J.; Singh, N. P. Tetrahedron
1994, 50, 6441-6493. (d) Scheffer, J. R.; Xia, W. Top. Curr. Chem. 2005,
254, 233-262.
(6) (a) Felix, G.; Lapouyade, R.; Bouas-Laurent, H.; Gaultier, J.; Hauw,
C. Tetrahedron Lett. 1975, 16, 409-412. (b) Gaultier, P. J.; Hauw, C.;
Bouas-Laurent, H. Acta Crystallogr. 1976, B32, 1220-1223. (c) Fernandez,
M.-J.; Gude, L.; Lorente, A. Tetrahedron Lett. 2001, 42, 891-893.
10.1021/jo060315i CCC: $33.50 © 2006 American Chemical Society
3
608
J. Org. Chem. 2006, 71, 3608-3611
Published on Web 03/16/2006

TABLE 1. Preparation of the Crystalline Salts
TABLE 2. Solid-State Photolysis of the Crystalline Saltsa
b
yield (%)
irrad. time
(h)
conv.
(%)
composition
1/acid
salt
ht-2
usy-ht-2
3
5
salt
solvent
1
1
1
1
1
1
1
ox
su
ad
fu
mu
ac•MeOH
ac
2
5
5
5
5
5
5
100
37
9
100
0
0
0
0
0
0
0
0
35
0
0
0
0
5
0
0
0
0
0
1
1
1
1
1
1
1
ox
su
ad
fu
MeOH
MeOH
i-PrOH
MeOH
MeOH
1:1
1:0.5
1:1
1:0.5
0
0
85
73
100
100
100
0
mu
ac•MeOH
1:0.5
0
51
0
18
MeOH
MeOH/i-PrOH (1:2)
1:0.5:0.5 MeOHa
1:0.5
ac
a
Pulverized salts were irradiated with a 400-W high-pressure mercury
a
MeOH is included in the crystal lattice.
b
lamp through a Pyrex filter under an Ar atmosphere at 3-7 °C. Yields
are based on the consumed reactant.
SCHEME 2. Solution Photolysis of 1a
1mu and 1ac‚MeOH underwent unsymmetrical dimerization in
the 9,10:4′,1′ cycloaddition manner. In both cases, the dimer
usy-ht-2 was selectively formed with high conversions. As will
be discussed later in more detail, this unsymmetrical photo-
dimerization of anthracene derivatives in the solid state is
unprecedented, while the analogous reactions are known to occur
9
,10
in solution.
On the contrary to the salt 1ac‚MeOH, the
corresponding salt without included MeOH (1ac) gave ht-2 and
3
, but no usy-ht-2.
Upon irradiation, anthracenes generally yield symmetric head-
to-tail or head-to-head dimers, that is, 9,10:10′,9′ or 9,10:9′,10′
4
,5
[
4+4] cycloadducts, respectively. In addition, they sometimes
undergo unsymmerical photodimerization to give 9,10:4′,1′ or
a
Product yields are based on the consumed reactant.
9
,10
9
,10:1′,4′ [4+4] cycloadducts
and 9,10:1′,2′ [4+2] cyclo-
4
a,b
adducts. Very recently, formation of a novel 1,2:2′,1′ [2+2]
of the anthrylmethyl radical intermediate photogenerated from
1
1
7
cycloadduct has been reported. In the solid state, however,
1
. The conversion of 1 was low (17%), probably because
only the symmetric [4+4] dimerization reactions have been
intramolecular electron transfer from the basic nitrogen to the
anthracene moiety quenched the responsible excited state. In
fact, in the presence of the added acid, ac (molar ratio ac/1 )
5
reported to date. Therefore, to the best of our knowledge,
formation of usy-ht-2 is the first example of the solid-state
unsymmetrical photodimerization of anthracenes.
0
.5), the conversion remarkably increased up to ∼100%
To rationalize the observed solid-state photoreactivity of the
salts, the X-ray diffraction studies were carried out. Suitable
single crystals could be obtained with 1ox, 1su, 1ad, 1mu, 1ac‚
MeOH, and 1ac. Their crystal structures (see Figure 1 and
Supporting Information) except that of 1ad consist of two
layers: one is formed from the anthracene moiety and another
from the dicarboxylic acid and the amino moiety. For 1ac‚
MeOH, the methanol molecule is included in the latter layer.
(
yield: ht-2, 41%; 3, 7%; 4, 9%).
Results for the solid-state photolysis are summarized in Table
2
. Free amine 1 recrystallized from i-PrOH was almost
5b
photoinert, as reported by Ihmels et al. Only a trace of
8
anthraquinone was produced by the photolysis (5 h). By
contrast, the salt 1ox underwent complete conversion to yield
the dimer ht-2 quantitatively. Thus, compared with the solution
photoreaction of 1 (Scheme 2), much more selective dimeriza-
tion was accomplished. In the case of 1su, 1ad, and 1fu, the
reactions were not as effective as 1ox. Lepidopterene (3) and a
reduced product 5 were produced in low yields from the salt
(9) The unsymmetrical intramolecular dimerizations of covalently linked
bisanthracenes, some of which proceed quantitatively, have been known.
(a) Felix, G.; Lapouyade, R.; Bouas-Laurent, H.; Clin, B. Tetrahedron Lett.
1976, 26, 2277-2278. (b) Castellan, A.; Desvergne, J.-P.; Bouas-Laurent,
1
su, whereas 1ad and 1fu showed no reactivity. Interestingly,
H. NouV. J. Chim. 1979, 3, 231-237. (c) Bouas-Laurent, H.; Castellan,
A.; Daney, M.; Desvergne, J.-P.; Guinand, G.; Marsau, P.; Riffaud, M.-H.
J. Am. Chem. Soc. 1986, 108, 315-317. (d) Desvergne, J.-P.; Bitit, N.;
Castellan, A.; Webb, M.; Bouas-Laurent, H. J. Chem. Soc., Perkin Trans.
2 1988, 1885-1894. (e) Marquis, M.; Desvergne, J.-P.; Bouas-Laurent, H.
J. Org. Chem. 1995, 60, 7984-7996.
(7) (a) Vermeersch, G.; Febvay-Garot, N.; Caplain, S.; Couture, A.;
Lablache-Combier, A. Tetrahedron Lett. 1979, 20, 609-612. (b) Couture,
A.; Lablache-Combier, A.; Lapouyade, R.; Felix, G. J. Chem. Res., Synop.
1
979, 258-259. (c) Adam, W.; Schneider, K.; Stapper, M.; Steenken, S. J.
Am. Chem. Soc. 1997, 119, 3280-3287. (d) Vadakkan, J. J.; Mallia, R. R.;
(10) The relatively newer intermolecular unsymmetrical anthracene
dimerization has also been reported, although the reactions were not
selective. (a) Fages, F.; Desvergne, J.-P.; Frisch, I.; Bouas-Laurent, H. J.
Chem. Soc., Chem. Commun. 1988, 1413-1415. (b) Becker, H.-D.; Becker,
H.-C.; Langer, V. J. Photochem. Photobiol., A 1996, 97, 25-32. (c) Noh,
T.; Lim, H. Chem. Lett. 1997, 495-496.
Prathapan, S.; Rath, N. P.; Unnikrishnan, P. A. Tetrahedron Lett. 2005,
4
6, 5919-5922.
8) We successfully solved the crystal structure of 1, but this was
published recently by Howie, R. A.; Wardell, S. M. S. V. Acta Crystallogr.
005, E61, o1686-o1688. The neighboring anthracene rings are not arranged
(
2
face-to-face, which is in agreement with its low photodimerization reactivity
in the solid state.
(11) Dolbier, W. R., Jr.; Zhai, Y.-A.; Battiste, M. A.; Abboud, K. A.;
Ghiviriga, I.; Wu, K. ARKIVOC 2006, iii, 97-103.
J. Org. Chem, Vol. 71, No. 9, 2006 3609

TABLE 3. Distances ds, du, and dL for the Nearest Faced
Anthracene Moieties
ds (Å)
du (Å)
dL (Å)
salt
A
B
A
B
A
B
1
1
1
1
1
1
ox
su
ad
mu
ac•MeOH
ac
3.77
3.93
4.84
4.78
4.85
3.94
3.86, 3.90
3.64, 3.64
3.90, 3.93
3.62, 3.68
3.58, 3.65
3.64, 3.67
3.62
3.85
4.51
4.89
4.86
3.92
a
5.26
3.79, 3.88
5.35
a
Salt 1ac•MeOH contains two independent molecules of 1 in the
asymmetric unit.
(
b) and (c), and only (b) is packed so as to yield ht-2 slowly,
5
b
while (a) and (c) are photoinert. From our own inspection of
their crystal packing, the aromatic acids in (a)-(c) are interacting
with 1 in a herringbone (C-H‚‚‚π) or stacking mode.
The distances between reacting points for symmetric (ds) and
unsymmetrical (du) dimerization and for the formation of 3 (dL)
are listed in Table 3. The value ds refers to the C9‚‚‚C10′ or
C10‚‚‚C9′ separation, du to the C9‚‚‚C4′ or C10‚‚‚C1′ separation,
and dL to the anthrylmethyl carbon‚‚‚C10′ separation. In the
salt 1ox, which yielded ht-2 quantitatively, the distance ds is
quite short (3.77 Å), and, hence, the normal symmetric
photodimerization may well occur efficiently. In other salts,
because ds is longer (>3.93 Å), the efficiency for the formation
of ht-2 should drop, thereby leading to the occurrence of other
reactions. Thus, for 1su and 1ac, where the distance dL is
relatively short (3.85 Å and 3.92 Å), lepidopterene (3) may be
formed through the competitive C-N bond homolysis followed
7
by the coupling of two anthrylmethyl radicals. For 1mu or
1
ac•MeOH, although both ds and dL are large, the distance du
is short (3.58-3.68 Å), resulting in the selective formation of
usy-ht-2. In 1ad, because the two anthracene moieties are
considerably separated from each other in the oblique direction,
all the distances, ds, du, and dL, are long. Hence, 1ad should be
photoinert.
The reason for 1mu and 1ac•MeOH to take the similar
packing pattern to yield usy-ht-2 is a profound issue. Both of
the molecules, mu and ac, are packed with an approximately
planar geometry. Although ac is spatially less-demanding than
mu (the intramolecular distances between the carboxylic carbons
for mu and ac are 6.32 and 4.13 Å, respectively), the methanol
molecule in the ac layer is making up for the room. Phenom-
enologically, we point out that the carbon chains of mu (diene)
and ac (alkyne) have higher unsaturation than that of other acids
(ox, su, ad, and fu; none, alkane, alkane, and alkene) and has
lower unsaturation than the Ihmels’ case (aromatics). Therefore,
it appears that the intermediate unsaturation of the acid
component brought about a higher chance of the unsymmetrical
photodimerization of 1, that is, a small lateral deviation from
the normal stacking, leading to the usy-ht-2 rather than ht-2
formation.
FIGURE 1. Crystal packing in the salts (a) 1ox, (b) 1mu, and (c)
ac•MeOH. Hydrogen atoms are omitted.
1
The packing behavior of the molecule 1 in the crystals of 1mu
and 1ac‚MeOH is very similar. In all the cases, there is a pair
of anthracene rings, which are arranged parallel and face each
other in a head-to-tail fashion. These situations are different
from the Ihmels’ salts ((a) 1‚4-nitrobenzoic acid, (b) 1‚2-
furancarboxylic acid, and (c) 1‚9-anthracenecarboxylic acid),
where the nearest anthracene rings are twisted to each other for
3
610 J. Org. Chem., Vol. 71, No. 9, 2006

Finally, we examined the thermal stability of usy-ht-2,
because the chemistry of intermolecular unsymmetrical an-
thracene dimers has rarely been reported. This compound
dissociated quantitatively to monomer 1 within 0.5 h at 100 °C
in DMSO-d6. From the kinetic plot, the first-order rate constant
Photoirradiation. Irradiation was carried out with a 400-W high-
pressure mercury lamp. A solution sample was placed in a Pyrex
test tube. After bubbling Ar gas for 0.5 h, the tube was sealed with
a rubber septum and was irradiated. A powdered solid sample was
sandwiched between two Pyrex plates and was set in our solid-
state photolysis vessel2 or in a polyethylene bag. In either case,
the irradiation was carried out under an Ar atmosphere. During
the irradiation, the samples were cooled by refrigerated circulating
water (3-7 °C).
a,b
3b,d
-
6 -1
at 40 °C was calculated as k ) 4.8 × 10 s . Therefore, usy-
ht-2 seems more labile than the corresponding unsymmetrical
photodimer of methyl 9-anthracenecarboxylate (t1/2 ∼ 3 h at
1
0c
1
00 °C in DMF).
In conclusion, we achieved the rapid and selective photo-
The yields of the photoproducts were estimated by measuring
1
the H NMR spectra of the reaction mixture immediately after the
dimerization of 1 in the solid state by utilizing salt formation
with particular dicarboxylic acids. The crystalline salt 1ox
yielded ht-2, and 1mu and 1ac‚MeOH yielded usy-ht-2
quantitatively. From the crystal structures of these salts, we
rationalized their solid-state reactivities.
photolysis. The products were separated by thin-layer or column
chromatography on silica gel. The data for the compound charac-
terization are summarized in Supporting Information.
Acknowledgment. We are indebted to Profs. Shigeru Ohba
(Keio University), Yohji Misaki (Ehime University), and Robert
J. McMahon (University of Wisconsin) for advice on performing
X-ray analysis and its refinement.
Experimental Section
Materials. The detailed synthetic procedure for 9-(N,N-dim-
ethylaminomethyl)anthracene (1) is described in Supporting Infor-
mation. All dicarboxylic acids and all solvents for crystallization
are available from commercial sources.
Supporting Information Available: Synthetic procedures for
Preparation of Crystalline Salts. A warm solution containing
1
and the crystalline salts. Spectral data of the products ht-2, usy-
1
and acid (molar ratio 1:0.5) in suitable solvent (Table 1) was
ht-2, 3, 4, and 5. Dissociation kinetics of usy-ht-2. The X-ray crystal
data and the CIF format for the salts 1ox, 1su, 1ad, 1mu,
allowed to stand at room temperature. After a few days, the
corresponding salt crystals appeared. They were collected by
filtration and dried in vacuo. The composition of the 1/acid of the
1
ac•MeOH, and 1ac and the dimer usy-ht-2. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
salt was determined on the basis of the H NMR and elemental
analysis. Further details are described in Supporting Information.
JO060315I
J. Org. Chem, Vol. 71, No. 9, 2006 3611